Vol 52, No 1 (2018)

A theoretical and experimental description of the synthesis of GaP nanowire crystals by molecularbeam epitaxy on Si(111) substrates with the use of gold as a catalyst is presented. The ratio between the fluxes of materials to be deposited and the substrate temperature are varied for a short time during nanowire synthesis in order to analyze the possibility of producing nanoinclusions of different polytypes. It is established that variations in the ratio between the fluxes of materials to be deposited and in the growth temperature bring about the controllable formation of inclusions, among them are structurally cubic crystalline regions. The inclusions are several nanometers thick.

The magnetic-field (0 T < B < 9 T) dependence of the longitudinal and Hall resistances at fixed temperatures (2 K < T < 50 K) for the HgCdTe/HgTe/HgCdTe system with a HgTe quantum well 20.3 nm in width are measured. The activation analysis of the magnetoresistance curves is used as a tool for identifying the mobility gaps between neighboring Landau levels. The activation-energy values obtained from the temperature dependences of the longitudinal resistance in the plateau regions of the quantum Hall effect with the filling factors ν = 1, 2, 3 make it possible to estimate the effective mass and the g factor of electrons in the system under study. Indications concerning the possibility of large values of the g factor (≅ 80) are obtained.

The quasi-mobility function of charge carriers with a specified energy for describing their dynamics using the kinetic equation is studied in the important case of two-term isotropic approximation. In the stationary case, the quasi-mobility function is independent of the source function of charge carriers and makes it possible to calculate the integral mobility. The correlation between the quasi-mobility and parameters of the system is analyzed. It is proved that this characteristic does not generally describe the contribution of charge carriers with a specified energy to the integral mobility. In the case of almost elastic scattering, the quasi-mobility, as is known, can have a clear physical meaning; however, in the case of the scattering of charge carriers at acoustic phonons in a solid, this quasi-mobility interpretation is found to be incorrect due to the specific features of the collision integral and the form of the quasi-mobility function.

The reflectance spectra of the topological insulator Pb_1–xSn_xSe (x = 0.2, 0.34) films grown by molecular-beam epitaxy on a ZnTe/GaAs substrate are measured in the range of 12–2500 cm^–1 at room temperature. Using dispersion analysis, the frequencies of transverse optical phonons, plasma frequencies, highfrequency permittivities, and layer thicknesses are determined. In the quasi-steady-state approximation, the interface mode frequencies of the four-layer structure are calculated as a function of the overlap parameter χ₁ (0 ≤ χ₁ ≤ 1). The parameter describes the degree of overlap of two interface modes localized at planes bounding the layer to the right and left. The existence of interacting interface modes in the structure makes its spectrum different from the sum of component spectra. These differences manifest themselves in the experiment. The conditions of the interaction of interface modes with IR radiation are discussed.

The influence of the Gd content on the semiconductor–metal phase transition in polycrystalline Sm_1–xGd_xS thin films produced by the flash evaporation of a powder in vacuum is studied. It is shown that the basic factor of the physical mechanism of the phase transition is compression of the film material. It is established that the films retain their semiconductor properties only up to a Gd content of x = 0.12.

The effect of the silicon-atom distribution profile in donor δ-layers of AlGaAs/InGaAs/AlGaAs heterostructures with donor–acceptor doping on the mobility of the two-dimensional electron gas is studied. The parameters of the δ-layer profiles are determined using the normal approximation of the spatial distributions of silicon atoms, measured by secondary-ion mass spectroscopy. It is shown that the standard deviation σ of the δ-layer profile can be reduced from 3.4 to 2.5 nm by the proper selection of growth conditions. Measurements of the magnetic-field dependences of the Hall effect and conductivity show that such a decrease in σ allowed an increase in the mobility of the two-dimensional electron gas in heterostructures by 4000 cm²/(V s) at 77 K and 600 cm²/(V s) at 300 K. The mobility calculation taking into account filling of the first two size-quantization subbands shows that an increase in the mobility is well explained by a reduction in the Coulomb scattering at ionized donors due to an increase in the effective thickness of the spacer layer with decreasing σ of the δ-layer profile.

The experimental and theoretical results of studies of optical absorption in doped Ge/Si quantumdot structures in the far-infrared region, corresponding to the energies of transitions of holes from the ground state to the lowest excited size-quantization state, are reported. An analytical theory of the size quantization of holes in a lens-shaped quantum dot is developed in the context of the adiabatic approximation with consideration for pair Coulomb interaction. It is shown that the interaction has no effect on the frequencies of lower interlevel resonances. This fact is representative of generalized Kohn’s theorem satisfied due to the specific geometric shape of the quantum dot. The experimental and theoretical values of the transition energies are in good agreement.

The influence of the electronegativity of phases, which is controlled by composite crystallization under the conditions of the effect of the electric discharge and covalence of piezophase cations, on the formation mechanism of the stable electret effect is determined. The specific features of the formation of the electret effect in composites based on polyolefins (HDPE, PP), fluorine-containing polymers (F42), and ferroelectric piezoelectric ceramics of the family of lead zirconate–titanate (Pb(Zr,Ti)O₃) crystallized under conditions of the effect of electric discharge plasma, are revealed. A physical model of electret composites taking into account the role of homocharges and heterocharges formed in a composite by its dispersion with piezoceramic particles of various structures—rhombohedral, tetragonal, and heterogeneous—is proposed.

X-ray diffraction and differential thermal analysis data obtained in the Cu_1.95Ni_0.05S phase-transition region are analyzed. It is established that the low-temperature rhombic α phase in Cu_1.95Ni_0.05S transforms to the hexagonal β phase at temperatures of 370–390 K and to the cubic γ phase at temperatures of 740–765 K according to the scheme \(\alpha \to \mathop {\alpha + \beta }\limits_{370 - 390K} \to \mathop {\alpha + \gamma }\limits_{740 - 765K} \to \gamma \). It is determined (using the temperature dependence of differential thermal analysis) that the transition α → β is accompanied by heat absorption while the transition β → γ is accompanied by heat release. It is found that both transitions are allowed and belong to the reconstructive type. Both transitions are found to occur in a fluctuation volume of ~10^–20 cm³ at temperature rates of 0.11 and 0.08 K^–1. It is demonstrated that the transition α → γ is accompanied by alternation of the structures passing through the intermediate β phase, which is incommensurate with respect to the α and γ phases.

An indium-phosphide InP sample subjected to the pore-generation procedure and then doped with S atoms is studied by the methods of X-ray diffraction analysis (XRD) and small-angle X-ray scattering (SAXS) (with CuK_α1-radiation). The XRD data demonstrate that the sample consists of (coherent) aligned homogeneous components. A point detector is used to obtain, in the anomalous transmission mode by Borrmann, a set of SAXS curves at sample positions varied by azimuthal rotations. The SAXS data are used to simulate a 2D SAXS pattern for the sample under study, which makes it possible to determine the long-distance translation symmetry and, consequently, the presence of a superstructure. The interplanar distances in the superstructure in the directions (110) and (1 0) of the InP lattice are found to be ~260 and 450 nm, respectively. The symmetry group of the superstructure is determined as C_2v in the (001) plane of the sample lattice.

New types of two-terminal tandem solar cells DSC/c-Si in which mesoscopic dye-sensitized solar cell (DSC) was connected in parallel with a crystalline silicon (c-Si) solar cell, were developed and investigated. We have measured the optical and photovoltaic parameters for both the individual and the fabricated tandem DSC/c-Si solar cells. It was shown that the highest efficiency of 14.7% for the tandem DSC/c-Si solar cell under standard AM1.5G (100 mW/cm²) illumination conditions was achieved for DSC based on 3.5 μm thick titanium dioxide photoelectrode.

The problem of the spatial localization of free electrons in 4H-SiC metal—oxide—semiconductor field effect transistors (MOSFETS) with an accumulation- and inversion-type n channel is theoretically analyzed. The analysis demonstrates that, in optimally designed accumulation transistors (ACCUFETs), the average distance from the surface, at which free electrons are localized, may be an order of magnitude larger than that in inversion MOSFETs. This can make 4H-SiC ACCUFETs advantageous as regards the effective carrier mobility in a conducting channel.

A method for visualization via atomic-force microscopy of the internal structure of photoactive layers of polymer solar cells using an ultramicrotome for photoactive layer cutting is proposed and applied. The method creates an opportunity to take advantage of atomic-force microscopy in structural investigations of the bulk of soft samples. Such advantages of atomic-force microscopy include a high contrast and the ability to measure various surface properties at nanometer resolution. Using the proposed method, samples of the photoactive layer of polymer solar cells based on a mixture of PTB7 polythiophene and PC₇₁BM fullerene derivatives are studied. The disclosed details of the bulk structure of this mixture allow us to draw additional conclusions about the effect of morphology on the efficiency of organic solar cells.

The results of crystallinity studies of an ensemble of cadmium-sulfide nanowire crystals synthesized from the vapor phase in a quasi-closed volume are reported. The nanowires are from 10 nm to several micrometers in diameter and tens of millimeters in length. Electron diffraction and luminescence are studied. The diffraction patterns and micrographs are presented. The results suggest a high degree of crystallinity of the nanowires.

The process of obtaining a modified structure for quantum cascade lasers is studied; this process includes growth using molecular-beam epitaxy, plasma etching, photolithography with the use of liquid etching, and the formation of special contacts for decreasing losses in the waveguide. The use of a special type of structure makes it possible, even without postgrowth overgrowth with a high-resistivity material, to attain parameters satisfying requirements to heterostructures in high-quality quantum cascade lasers at maximal simplification of the entire preparation process.